The present invention relates to an apparatus for molding a preform which is used in making an information recording disc, and particularly to such an apparatus which provides for automatic transfer of the preform from a molding cup to a disc molding press.
Compression molded recorded discs are generally made by forming a preform of thermoplastic material and placing the preform centrally between a pair of heated molds. The molds are closed against the preform, melting the thermoplastic and compressing it, causing it to flow outwardly to fill the mold cavity and define the record contour. In molding the discs it is highly desirable to prevent, or at least minimize, surface defects, such as voids caused by trapped air and gasses, which can disrupt the grooves in the surface of the disc. The prevention of such surface defects is more desirable in making high density information discs, such as video discs, which utilize packing densities of the order of 10,000 grooves per inch, since even the smallest void can disrupt a large number of the grooves.
The preforms used to mold the discs are generally formed at the molding machine in a cup-shaped mold, generally referred to as a "cup". The heated plastic material is injected into the cup through an opening in the side of the cup until the cup is filled with the plastic. When the heated plastic is injected into the cup it extrudes in a snake-like fashion across the interior of the cup and then swirls around in the cup until the cup is filled. This results in a non-uniform temperature distribution across the plastic material which, along with the swirling action, can result in air and gasses being trapped in the preform and in variations in density of the preform. The trapped air and gasses and the variation in density can result in defects in the recorded disc formed from the preform. In addition, the side injection of the plastic into the cup can result in decomposition of the molding composition due to shear, particularly at the edge of the preform adjacent to the injection opening in the cup. This also can adversely affect the quality of the record made from the preform.
To form a preform in which the above defects are eliminated or at least minimized, there has been developed a mold cup having a pair of mold members, one of which can fit inside the mold cavity of the other and wherein the plastic material inlet opening is in the bottom of one of the mold members. To mold a preform in this mold cup, the two mold members are moved together until one fits inside the other and the volume defined by the cavities in the mold members is considerably smaller than that of the desired preform. The plastic material is injected into the reduced volume of the mold cavities until they are filled. Continued injection of the plastic material causes the upper mold member to move away until the desired volume of the mold cavity is achieved. That volume is determined by a preset control which shuts off the flow of the plastic into the mold cavity. The density and shape of the preform is controlled by a force exerted upon the upper mold member which is less than, and which opposes, the force of the incoming plastic material. The mold members are then separated to permit removal of the preform. To mold records in an automatic molding press it is desirable to have a mold cup which performs in the above manner but which also provides for automatic transfer of the preform from the mold cup to the record mold press.